Encapsulation of CsPb2Br5 in TiO2 Microcrystals to Enhance Environmental Stability

All-inorganic lead halide perovskite has emerged as an attractive semiconducting material due to its unique optoelectronic properties. However, its poor environmental stability restricts its broad application. Here, a simple method for the fabrication of CsPb2Br5/TiO2 is investigated. The introduction of p-aminobenzoic acid, which has two functional groups, is critical for the capping of amorphous TiO2 on CsPb2Br5. After calcination at 300 °C, amorphous TiO2 crystallizes into the anatase phase. The CsPb2Br5/TiO2 NCs show high long-term stability in water and enhanced stability against ultraviolet radiation and heat treatment, owing to the chemical stability of TiO2. More importantly, photo-electrochemical characterizations indicate that the formation of TiO2 shells can increase the charge separation efficiency. Hence, CsPb2Br5/TiO2 exhibits improved photoelectric activity owing to the electrical conductivity of the TiO2 in water. This study provides a new route for the fabrication of optoelectronic devices and photocatalysts based on perovskite NCs in the aqueous phase. Furthermore, the present results demonstrate that CsPb2Br5/TiO2 NCs has considerable potential to be used as a photoelectric material in optical sensing and monitoring.


Introduction
Colloidal semiconductor nanocrystals (NCs) possess a unique blend of quantum size effects that effectively amplify their optical properties in comparison to their volumetric counterparts; they have emerged as a promising photoelectric material that is currently under extensive investigation.These NCs typically exhibit a size ranging from 2 to 20 nm, possess a distinctive surface chemistry, and exist in a state of colloidal freedom, enabling their dispersion in diverse solvents and substrates and ultimately facilitating their integration into a wide array of devices.Over the past few years, significant advances have been made in the field of all-inorganic perovskite nanocrystal synthesis, with extensive research being conducted in this domain.As a result, all-inorganic cesium lead halide perovskite NCs, offering narrow emission bands, excellent charge transport properties, high quantum efficiencies, and broad color tunability, have been developed [1][2][3][4].Therefore, lead halide perovskite NCs have been regarded as the most promising materials for application in photodetectors [5], lasers [6], photovoltaics [7], and LEDs [8].
Although strong progress has been made in the preparation of all-inorganic perovskite nanocrystals, many challenges remain.For example, perovskites show great promise in terms of photoluminescent quantum yields and color tunability; however, the poor chemical stability of the perovskite, including poor water, photo, and thermal stability, severely limits their application.Thus, it is critical to achieve environmental stability for perovskite NCs in practical applications and highly desirable to develop methods that can prepare perovskite NCs with tunable morphologies and enhanced stabilities.In order to improve the stability, perovskite NCs have been embedded in silicone resin [9], polymer [10], or inorganic matrix [11][12][13][14], or coated with strongly binding ligands [15].Although the chemical stability of perovskite NCs can be enhanced significantly via these methods, the insulating shells (including organics or amorphous inorganic shells) restrict charge transport for further optoelectronic or catalytic applications.TiO 2 is classified as an N-type semiconductor photocatalyst, which has the advantages of high optical stability, low cost, and non-toxicity; hence, it has been widely studied in the field of energy and environment.TiO 2 has a wide band gap energy of 3.2 eV, so photons with the appropriate energy (<400 nm) can excite electrons to jump from the valence band to the conducting band, resulting in electrons (e − ) and holes (h + ).Therefore, as an excellent electron collecting and transporting material, TiO 2 is an excellent shell material to enhance the stability of perovskites.Zheng's group has synthesized CsPbBr 3 NCs coated with anatase TiO 2 via hydrolysis and calcination processes, and the CsPbBr 3 /TiO 2 NCs exhibited improved chemical stability [16].Meanwhile, a photoelectrochemical study indicated that the formation of CsPbBr 3 /TiO 2 could facilitate the charge transfer of CsPbBr 3 NCs.During the formation of CsPbBr 3 /TiO 2 , tetrabutyl titanate (TBT) was added into the CsPbBr 3 solution directly.However, perovskite NCs are easily degraded in polar environments.Thus, the strong polarity of TBT can affect the stability of perovskite NCs during the coating progress, due to their high sensitivity to the surrounding environment.In addition, this method cannot eliminate the presence of water in the fabrication of TiO 2 -coated CsPbBr 3 NCs.In the field of photocatalysis, Tüysüz et al. fabricated a CsPbBr 3 /P25 composite for photocatalytic benzyl alcohol oxidation and proved its higher visible catalytic activity than P25 [17].However, the existence of CsPbBr 3 on the surface of P25 is not conducive to the stability of CsPbBr 3 .Thus, it is essential to develop a novel strategy to enhance the chemical stability of perovskite NCs as well as maintain good charge transport properties.
Due to its unique photoelectric performance, CsPb 2 Br 5 has attracted the attention of researchers.Compared with CsPbBr 3 (with a 3D structure), CsPb 2 Br 5 (with a 2D structure) shows excellent optical properties due to its sufficient gain toward whispering gallery mode lasing [18,19].Furthermore, CsPb 2 Br 5 displays a stronger stability against water compared with CsPbBr 3 .Nonetheless, its stability against water also limits its broad application.Although different methods have been reported to enhance the stability of perovskite NCs, all of them have focused on improving the stability of CsPbBr 3 .There is no report on how to improve the environmental stability of CsPb 2 Br 5 .
In this study, we report a facile method to fabricate TiO 2 -coated CsPb 2 Br 5 NCs.The ligands, palmitic acid (PA), and PABA (which have two functional groups including the carboxyl group and amino group) were introduced to the fabrication process.Titanium carboxylate was first formed by the reaction of the carboxyl group in PABA and tetrabutyl titanate (TBT).The introduction of PABA avoids the negative effect of TBT, which has strong polarity.Furthermore, the CsPb 2 Br 5 nanocrystals can be capped by amorphous TiO 2 (A-TiO 2 ) via the interaction between the amino group and CsPb 2 Br 5 NCs.After calcination, the A-TiO 2 shell is crystallized into the anatase crystal phase.The formation of the protective TiO 2 shells can isolate NCs from water, even when immersing the CsPb 2 Br 5 /TiO 2 nanocrystals in water.Meanwhile, the photostability and thermal stability are also significantly improved.Moreover, the charge transfer and photoelectric activity in real water were also tested.

Preparation of CsPb 2 Br 5 /TiO 2 NCs
To fabricate CsPb 2 Br 5 /TiO 2 NCs, the synthesis process involved several steps.Firstly, the Cs-precursor was fabricated.As a typical procedure, Cs 2 CO 3 (0.08 g) was loaded into a 50 mL 3-neck flask along with octadecene (3 mL) and stearic acid (0.55 g), heated for 0.5 h at 120 • C, and then heated under N 2 to 150 • C until all Cs 2 CO 3 was completely dissolved.Then, the product was kept in sample bottles.In order to prevent Cs-stearate from precipitating out of the ODE at room temperature, the solution needed to be preheated to 120 • C before injection.In a separate 3-necked flask, 5 mL ODE and 0.068 g PbBr 2 were mixed together.The flask was then heated to 120 • C for 1 h under a nitrogen flow.Following this, 0.5 g palmitic acid and 0.5 mL OAm were injected into the solution.The temperature was then increased to 160 • C. At this point, 0.4 mL of the previously prepared Cs-precursor solution was quickly injected into the PbBr 2 solution.To fabricate CsPb 2 Br 5 /TiO 2 NCs, the TiO 2 precursor (which was fabricated by the mixing of ODE (4 mL), TBT (0.5 mL), and PABA (0.05 g)) was injected into the solution as soon as possible after the injection of Cs-precursor.The reaction was maintained for 10 min followed by cooling in an ice bath.Then, the as-prepared crude solution was centrifuged and purified 3 times by n-hexane.The precipitate after centrifugation was dried in a vacuum at 30 • C. Finally, the product was calcined under nitrogen flow at 300 • C for 4 h to obtain the desired CsPb 2 Br 5 /TiO 2 NCs.

Material Characterization
The morphologies of the samples were determined by using a Tecnai F20 S-TWIN microscope transmission electron microscope operating at 200 kV (TEM, FEI Company, Hillsboro, OR, USA).The crystal structures of the powders were investigated on a Rigaku D/MAX-2400 diffractometer (Rigaku, Tokyo, Japan) with Cu-Ka radiation, operated at 40 kV and 40 mA (scanning step: 0.02 • /s) in the 2θ range of 10-80 • .FT-IR spectra of the samples were characterized using the FT-IR method (NICOLET 6700, Thermo SCIENTIFIC, Waltham, MA, USA).The XPS patterns were acquired by an X-ray photoelectron spectrometer (ESCALAB™ 250Xi, Thermo Fisher, Waltham, MA, USA).The PL spectra of the perovskite NCs were obtained using the F-7000 Fluorescence Spectrophotometer (Hitachi, Hitachi City, Japan).

Environmental Stability Characterization
Water stability: The perovskite NCs were immersed in DI and dispersed evenly by ultrasound.The CsPb 2 Br 5 /TiO 2 NCs with different soaking times were dried and the PL spectra were measured.
Photostability: The perovskite NCs were subjected to ultraviolet light irradiation.The experiment involved taking powder samples at regular intervals, starting from the 12th hour and continuing until the 24th hour.These samples were later used for PL spectra testing.
Thermal stability: To investigate the influence of temperature on the luminescence properties of perovskite NCs, a series of experiments were conducted.The perovskite NCs were subjected to heating in a controlled environment with temperatures ranging from 30 to 200 • C. To obtain a comprehensive understanding of the temperature-dependent luminescence behavior, the PL spectra of the perovskite NCs were measured at regular intervals of 10 • C.This systematic approach allowed for analysis of how changes in temperature affect the emission characteristics of the perovskite NCs.By examining the PL spectra obtained at different temperatures, valuable insights into the thermal stability and performance of perovskite NCs, in terms of luminescence, can be obtained.

Results and Discussion
First, the TiO 2 precursor was fabricated from ODE, TBT, and PABA.In terms of hard/soft acid/base theory [20], the -COOH tends to react with Ti 4+ to form carboxylate.On the other hand, the -NH 2 group in PABA does not participate in the reaction with Ti 4+ .Therefore, in the presence of carboxylate, the Ti 4+ ions can interact with the perovskite through the -NH 2 groups when the carboxylate is introduced into the solution of lead halide perovskite precursor at a temperature of 160 • C.This interaction between Ti 4+ and the perovskite leads to the formation of a composite material known as A-TiO 2 .Meanwhile, the excess of PbBr 2 is generated by the addition of carboxylate, which may impede the diffusion of PbBr 2 in solution.Hence, CsPb 2 Br 5 is formed by the presence of an excess of PbBr 2 [21].As a result, CsPb 2 Br 5 /A-TiO 2 is synthesized (Scheme 1).After calcination for 4 h at 300 • C under a nitrogen atmosphere, A-TiO 2 is transformed into anatase phase.EDAX was performed for the elemental analysis TEM and HR-TEM were utilized in this study to investigate the evolution of CsPb2Br5 and TiO2. Figure 1b shows that the CsPb2Br5 NCs have a core size of around 10 nm and are embedded in a matrix.Furthermore, the HR-TEM image (Figure 1c) of the CsPb2Br5 NCs reveals that the interplanar distance is about 0.30 nm, corresponding to the (213) plane of the crystal.Meanwhile, the shell without lattice fringe can be ascribed to the formation of A-TiO2.In addition, the obtained sample (CsPb2Br5/A-TiO2) still shows strong photoluminescence under ultraviolet radiation.However, if the mixture of TBT and ODE without PABA participates in the reaction, the photoluminescence of the resulting product disappears (Figure 1d).Hence, the introduction of PABA is beneficial to reduce the negative effect of TBT polarity on perovskite NCs.After calcination at 300 °C, the interplanar distance of CsPb2Br5 is still 0.30 nm, indicating that the crystal phase of perovskite does not change (Figure 1f).Thus, we can deduce that the formation of anatase TiO2 has no The low intensity of the anatase TiO 2 XRD peaks indicates incomplete crystallization of TiO 2 with possible amorphous components [16].The broad XRD peak is caused by the formation of carbon during the heating process under a nitrogen atmosphere.
The crystal structures of pure TiO 2 and CsPb 2 Br 5 have been extensively studied and are presented in Figure S2.These crystal structures provide valuable insights into the properties and behaviors of these materials.The results of the study confirm that TiO 2 adopts the anatase crystal structure, while CsPb 2 Br 5 exhibits a tetragonal phase structure.
The tetragonal phase structure of CsPb 2 Br 5 is known for its stability and ability to withstand external pressures.To better understand the formation of TiO2 shells of CsPb2Br5 NCs, Fourier transform infrared (FTIR) spectra of the ligand PABA, CsPb2Br5/A-TiO2, and CsPb2Br5/TiO2 NCs were obtained and shown in Figure 2a.The first FTIR spectrum corresponds to the ligand PABA, where peaks assigned to N-H vibrations can be observed at 3364 and 3459 cm −1 [20].These peaks indicate the presence of the N-H functional group in PABA.Moving on to the FTIR spectrum of CsPb2Br5/A-TiO2, the strong C-H stretching bands at 2918 and 2846 cm −1 confirm the presence of PA/OAm ligands [23,24].Additionally, the characteristic peaks assigned to N-H vibrations are still present in CsPb2Br5/A-TiO2, albeit with a decrease in intensity compared to PABA.This reduction in N-H peaks can be attributed to the interaction between NH2 and CsPb2Br5.Notably, the characteristic peak of -COOH is still not observed in this spectrum.Combining observations from the TEM analysis and FTIR spectra, it can be concluded that CsPb2Br5 NCs are indeed coated with A-TiO2.Upon calcination, the FTIR spectrum of CsPb2Br5/TiO2 shows a significant decrease in the characteristic peaks associated with different surface ligands.This decrease is attributed to decomposition of the ligands during the calcination process.
X-ray photoelectron spectroscopy (XPS) measurements were further carried out to study the constituent compositions and chemical states of CsPb2Br5/TiO2 NCs.The results of the XPS analysis are shown in Figure 2b-e: the signals of Pb, Cs, Ti, and O elements are clearly illustrated.To calibrate the binding energies, the reference C 1s peak at 284.6 eV was used.As shown in Figure 2b, the binding energy peaks at 737.5 and 723.5 eV can be assigned to Cs 3d3/2 and Cs 3d5/2, respectively [25,26]. Figure 2c shows the high-resolution XPS spectrum of the Pb 4f core level.The two spectral peaks with binding energies of 137.9 and 142.8 eV correspond to Pb 4f7/2 and Pb 4f5/2 levels.
The Ti 2p peaks can be fitted into four bands at 457.8, 460.0, 463.2, and 464.2 eV, corresponding to Ti 3+ 2p3/2, Ti 4+ 2p3/2, Ti 3+ 2p1/2, and Ti 4+ 2p1/2 [27,28].Moreover, the high-resolution spectrum of O 1s is also obtained.As shown in Figure 2e, the O 1s main peaks at 529.2 and 530.8 eV are indexed to the Ti-O bonds and Ti-OH groups, which is consistent with the binding energy of O 2− in the TiO2 lattices [29].TEM and HR-TEM were utilized in this study to investigate the evolution of CsPb 2 Br 5 and TiO 2 .Figure 1b shows that the CsPb 2 Br 5 NCs have a core size of around 10 nm and are embedded in a matrix.Furthermore, the HR-TEM image (Figure 1c) of the CsPb 2 Br 5 NCs reveals that the interplanar distance is about 0.30 nm, corresponding to the (213) plane of the crystal.Meanwhile, the shell without lattice fringe can be ascribed to the formation of A-TiO 2 .In addition, the obtained sample (CsPb 2 Br 5 /A-TiO 2 ) still shows strong photoluminescence under ultraviolet radiation.However, if the mixture of TBT and ODE without PABA participates in the reaction, the photoluminescence of the resulting product disappears (Figure 1d).Hence, the introduction of PABA is beneficial to reduce the negative effect of TBT polarity on perovskite NCs.After calcination at 300 • C, the interplanar distance of CsPb 2 Br 5 is still 0.30 nm, indicating that the crystal phase of perovskite does not change (Figure 1f).Thus, we can deduce that the formation of anatase TiO 2 has no effect on the crystal phase of the CsPb 2 Br 5 .In addition, the lattice fringe of TiO 2 (0.35 nm) belonging to the (101) plane of anatase TiO 2 is observed in the TEM image (Figure 1e,f).
Selective electron diffraction (SAD) was also performed to investigate the structures of CsPb 2 Br 5 and CsPb 2 Br 5 /TiO 2 NCs.As shown in Figure S3, the SAD image of CsPb 2 Br 5 revealed clear rings corresponding to the (213) plane.This indicates that the crystal structure of CsPb 2 Br 5 was well-defined.However, the typical plane ascribed to anatase TiO 2 was not observed in the SAD image of CsPb 2 Br 5 /TiO 2 NCs.This result may be caused by the incomplete crystallization of TiO 2 .This observation is consistent with the XRD pattern, which also shows a lack of distinct peaks corresponding to anatase TiO 2 .The incomplete crystallization of TiO 2 in CsPb 2 Br 5 /TiO 2 NCs may be attributed to various factors such as the synthesis conditions or the presence of CsPb 2 Br 5 , which could inhibit the formation of TiO 2 crystals.Further investigations are needed to fully understand the reasons behind the incomplete crystallization of TiO 2 in CsPb 2 Br 5 /TiO 2 NCs.By improving the crystallization of TiO 2 , the overall performance and properties of CsPb 2 Br 5 /TiO 2 NCs can potentially be enhanced for various applications in optoelectronics and photocatalysis.
To better understand the formation of TiO 2 shells of CsPb 2 Br 5 NCs, Fourier transform infrared (FTIR) spectra of the ligand PABA, CsPb 2 Br 5 /A-TiO 2 , and CsPb 2 Br 5 /TiO 2 NCs were obtained and shown in Figure 2a.The first FTIR spectrum corresponds to the ligand PABA, where peaks assigned to N-H vibrations can be observed at 3364 and 3459 cm −1 [20].These peaks indicate the presence of the N-H functional group in PABA.Moving on to the FTIR spectrum of CsPb 2 Br 5 /A-TiO 2 , the strong C-H stretching bands at 2918 and 2846 cm −1 confirm the presence of PA/OAm ligands [23,24].Additionally, the characteristic peaks assigned to N-H vibrations are still present in CsPb 2 Br 5 /A-TiO 2 , albeit with a decrease in intensity compared to PABA.This reduction in N-H peaks can be attributed to the interaction between NH 2 and CsPb 2 Br 5 .Notably, the characteristic peak of -COOH is still not observed in this spectrum.Combining observations from the TEM analysis and FTIR spectra, it can be concluded that CsPb 2 Br 5 NCs are indeed coated with A-TiO 2 .Upon calcination, the FTIR spectrum of CsPb 2 Br 5 /TiO 2 shows a significant decrease in the characteristic peaks associated with different surface ligands.This decrease is attributed to decomposition of the ligands during the calcination process.In order to investigate the influence of TiO2 on the PL characteristics of perovskite NCs, a comprehensive analysis of the PL spectra was conducted before and after the introduction of TiO2.By measuring the PL spectra, we aimed to assess any changes in the intensity, peak position, and overall emission behavior of the perovskite NCs following the addition of TiO2.As shown in Figure 3a, compared with perovskite NCs without TiO2, the PL intensity of solid CsPb2Br5/TiO2 NCs decreases.This decrease in PL intensity can be attributed to the transfer of electrons from the conduction band (CB) of CsPb2Br5 NCs to that of TiO2.This phenomenon supports the potential use of CsPb2Br5/TiO2 NCs in optoelectronic applications [16,30].Hence, it is reasonable that the CsPb2Br5/TiO2 NCs can be used in optoelectronics.In addition, the problem of the instability of perovskite NCs often restricts their application.Thus, the environmental stability of CsPb2Br5 NCs embedded in TiO2, including water stability, photostability, and thermal stability, were studied.These properties are crucial for the long-term performance and reliability of optoelectronic devices utilizing CsPb2Br5/TiO2 NCs.The findings from these studies can provide valuable insights into the feasibility and suitability of CsPb2Br5/TiO2 NCs for various optoelectronic applications.2b, the binding energy peaks at 737.5 and 723.5 eV can be assigned to Cs 3d 3/2 and Cs 3d 5/2 , respectively [25,26]. Figure 2c shows the high-resolution XPS spectrum of the Pb 4f core level.The two spectral peaks with binding energies of 137.9 and 142.8 eV correspond to Pb 4f 7/2 and Pb 4f 5/2 levels.
In order to investigate the influence of TiO 2 on the PL characteristics of perovskite NCs, a comprehensive analysis of the PL spectra was conducted before and after the introduction of TiO 2 .By measuring the PL spectra, we aimed to assess any changes in the intensity, peak position, and overall emission behavior of the perovskite NCs following the addition of TiO 2 .As shown in Figure 3a, compared with perovskite NCs without TiO 2 , the PL intensity of solid CsPb 2 Br 5 /TiO 2 NCs decreases.This decrease in PL intensity can be attributed to the transfer of electrons from the conduction band (CB) of CsPb 2 Br 5 NCs to that of TiO 2 .This phenomenon supports the potential use of CsPb 2 Br 5 /TiO 2 NCs in optoelectronic applications [16,30].Hence, it is reasonable that the CsPb 2 Br 5 /TiO 2 NCs can be used in optoelectronics.In addition, the problem of the instability of perovskite NCs often restricts their application.Thus, the environmental stability of CsPb 2 Br 5 NCs embedded in TiO 2 , including water stability, photostability, and thermal stability, were studied.These properties are crucial for the long-term performance and reliability of optoelectronic devices utilizing CsPb 2 Br 5 /TiO 2 NCs.The findings from these studies can provide valuable insights into the feasibility and suitability of CsPb 2 Br 5 /TiO 2 NCs for various optoelectronic applications.
chines 2023, 14, x FOR PEER REVIEW 8 of 10 helps to enhance their resistance to water degradation.This finding suggests that the incorporation of TiO2 into perovskite NCs represents a promising strategy to improve their water stability.The photostability of CsPb2Br5/TiO2 powder was also monitored.As shown in Figure 3d, the photostability test result demonstrates that CsPb2Br5/TiO2 NCs can maintain their original PL intensity even under UV light for 24 h.Additionally, another drawback of the application of CsPb2Br5 NCs is their instability at elevated temperatures.Figure 3e indicates that when the treatment temperature was increased from 30 to 200 °C, the PL intensity of CsPb2Br5/TiO2 was essentially consistent with the original PL intensity; that is, the PL intensity does not decrease with increasing temperature.However, the PL intensity of CsPb2Br5/A-TiO2 decreased to about 10% when the temperature was increased to 100 °C.This result further indicates that the TiO2 shell can protect the CsPb2Br5 NCs effectively.The enhanced stability of these nanocrystals in different environmental conditions further strengthens their potential in the field.Future research efforts can focus on optimizing the stability and performance of CsPb2Br5/TiO2 NCs for practical applications in optoelectronics.Based on the above discussion, the stability of CsPb2Br5 can be enhanced by coating with TiO2.This is due to the electrical conductivity of TiO2, which allows for better charge transport properties.In order to further study the photo-electrochemical characterization, a comparison was made between TiO2 and CsPb2Br5/TiO2 NCs using a 405 LED light (Zhonghe, Gauangzhou, China) in a 0.5 M K2SO4 aqueous solution.In Figure 3f, the transient photocurrent responses are shown, indicating that the cathodic photocurrent of CsPb2Br5/TiO2 NCs is approximately three times larger than that of pure TiO2.This suggests that the introduction of CsPb2Br5 enhances the photoelectronic properties of TiO2.Under UV light radiation, electron-hole pairs are formed in TiO2, allowing for photo-responsive behavior in water.However, after the introduction of CsPb2Br5, additional electrons can be formed under irradiation in CsPb2Br5 and then transported to TiO2.This leads to a significantly higher photocurrent in CsPb2Br5/TiO2 compared to pure TiO2.Overall, the coating of TiO2 on CsPb2Br5 not only enhances the stability of CsPb2Br5 but also improves its photoelectronic properties.This finding highlights the potential of CsPb2Br5/TiO2 NCs in various applications, such as solar cells and photocatalysis.As shown in Figure 3b,c, the PL intensity of CsPb 2 Br 5 /TiO 2 NCs and CsPb 2 Br 5 /A-TiO 2 NCs did not show a significant decrease within 10 h of immersion in water.It is worth noting that the water-resistance stability is much higher than that of CsPb 2 Br 5 /A-TiO 2 .When the soaking time in water was increased to 96 h, the PL intensity of CsPb 2 Br 5 /TiO 2 NCs was essentially the same as the original PL intensity.This suggests that the water stability of CsPb 2 Br 5 /TiO 2 NCs is maintained even after prolonged exposure to water.The PL intensity of CsPb 2 Br 5 /A-TiO 2 NCs decreased to less than 20% that of the original PL intensity.On the other hand, the PL intensity of CsPb 2 Br 5 /A-TiO 2 NCs decreased to less than 20% of the original PL intensity.This significant decrease in PL intensity indicates that CsPb 2 Br 5 /A-TiO 2 NCs are not as water-stable as CsPb 2 Br 5 /TiO 2 NCs.These results highlight the importance of the formation of TiO 2 under calcination conditions when improving the water stability of perovskite NCs.The presence of TiO 2 in CsPb 2 Br 5 /TiO 2 NCs helps to enhance their resistance to water degradation.This finding suggests that the incorporation of TiO 2 into perovskite NCs represents a promising strategy to improve their water stability.The photostability of CsPb 2 Br 5 /TiO 2 powder was also monitored.
As shown in Figure 3d, the photostability test result demonstrates that CsPb 2 Br 5 /TiO 2 NCs can maintain their original PL intensity even under UV light for 24 h.Additionally, another drawback of the application of CsPb 2 Br 5 NCs is their instability at elevated temperatures.Figure 3e indicates that when the treatment temperature was increased from 30 to 200 • C, the PL intensity of CsPb 2 Br 5 /TiO 2 was essentially consistent with the original PL intensity; that is, the PL intensity does not decrease with increasing temperature.However, the PL intensity of CsPb 2 Br 5 /A-TiO 2 decreased to about 10% when the temperature was increased to 100 • C.This result further indicates that the TiO 2 shell can protect the CsPb 2 Br 5 NCs effectively.The enhanced stability of these nanocrystals in different environmental conditions further strengthens their potential in the field.Future research efforts can focus on optimizing the stability and performance of CsPb 2 Br 5 /TiO 2 NCs for practical applications in optoelectronics.
Based on the above discussion, the stability of CsPb 2 Br 5 can be enhanced by coating with TiO 2 .This is due to the electrical conductivity of TiO 2 , which allows for better charge transport properties.In order to further study the photo-electrochemical characterization, a comparison was made between TiO 2 and CsPb 2 Br 5 /TiO 2 NCs using a 405 LED light (Zhonghe, Gauangzhou, China) in a 0.5 M K 2 SO 4 aqueous solution.In Figure 3f, the transient photocurrent responses are shown, indicating that the cathodic photocurrent of CsPb 2 Br 5 /TiO 2 NCs is approximately three times larger than that of pure TiO 2 .This suggests that the introduction of CsPb 2 Br 5 enhances the photoelectronic properties of TiO 2 .Under UV light radiation, electron-hole pairs are formed in TiO 2 , allowing for photoresponsive behavior in water.However, after the introduction of CsPb 2 Br 5 , additional electrons can be formed under irradiation in CsPb 2 Br 5 and then transported to TiO 2 .This leads to a significantly higher photocurrent in CsPb 2 Br 5 /TiO 2 compared to pure TiO 2 .Overall, the coating of TiO 2 on CsPb 2 Br 5 not only enhances the stability of CsPb 2 Br 5 but also improves its photoelectronic properties.This finding highlights the potential of CsPb 2 Br 5 /TiO 2 NCs in various applications, such as solar cells and photocatalysis.

Conclusions
In this work, we aimed to investigate the impact of TiO 2 on the environmental stability of CsPb 2 Br 5 .The experimental findings reveal a significant enhancement in the stability of CsPb 2 Br 5 against various environmental factors, including water, heating, and photo radiation.This improvement in stability can be attributed to the incorporation of TiO 2 .Furthermore, encapsulating CsPb 2 Br 5 NCs into anatase TiO 2 (which has excellent electrical conductivity) enables the photoinduced charge in perovskite to transfer efficiently.Compared to pure TiO 2 , CsPb 2 Br 5 /TiO 2 shows higher photoelectric activity in water testing.These results have important implications for the development of optoelectronic devices and visible light photocatalysts.The incorporation of TiO 2 not only enhances the stability of CsPb 2 Br 5 but also improves its photoelectric performance.This work provides valuable insights and guidelines for the preparation of advanced optoelectronic materials.
Future research into these new nanocrystals (NCs) should be directed towards exploring their vast potential in optoelectronic applications.Specifically, researchers should concentrate on harnessing the unique properties of these NCs to develop advanced technologies such as lasers, light-emitting diodes (LEDs), photovoltaics, optical sensing systems, and photon detection devices.

Figure 3 .
Figure 3. (a) PL spectra of perovskite before (black line) and after (blue line) the addition of TiO2, (b,c) PL intensity and relative PL intensity of CsPb2Br5/A-TiO2 and CsPb2Br5/TiO2 NCs powders after immersion in water, (d,e) photostability and thermal stability of CsPb2Br5/A-TiO2 and CsPb2Br5/TiO2 NCs, and (f) photocurrent response of TiO2 and CsPb2Br5/TiO2.
Figure S3: SAD images of the (a) CsPb 2 Br 5 /A-TiO 2 and (b) CsPb 2 Br 5 / TiO 2. Author Contributions: Y.W. (Yuezhu Wang): investigation, formal analysis, data curation, writingoriginal draft, funding acquisition.X.X.: investigation, methodology, formal analysis, data curation, writing-original draft, writing-review & editing, project administration, funding acquisition.W.Y.: investigation, data curation.Y.W. (Yawen Wei): data curation.J.W.: formal analysis, data curation, writing-original draft.All authors have read and agreed to the published version of the manuscript.Funding: The study was supported by the Open Foundation of the Key Laboratory of Industrial Ecology and Environmental Engineering MOE (KLIEEE-21-09), the Doctoral Research Project of the National Marine Environmental Monitoring Center (2022-Z-307), the Fellowship of China Postdoctoral Science Foundation (2021M700653), Xinghai Project of Dalian Maritime University (XHGC-04-21019), Doctoral research Project of Liaoning Natural Science Foundation (2022BS098), National Natural Science Foundation of China (52171343), Dalian key field innovation team (2021RT05).